The present invention relates generally to gas generators, those used in inflatable occupant safety restraints in motor vehicles and aircraft for example, and more particularly to reducing the toxicity of effluent gases produced by nitrogen-containing gas generating compositions.
Inflatable occupant restraint devices for motor vehicles have been under development worldwide for many years, including the development of gas generating compositions for inflating such occupant restraints. Because the inflating gases produced by the gas generants must meet strict toxicity requirements, many gas generants now in use are based on alkali or alkaline earth metal azides, particularly sodium azide. When reacted with an oxidizing agent, sodium azide forms a relatively nontoxic gas consisting primarily of nitrogen.
However, azide-based gas generants are inherently volatile to handle and entail relatively high risk in manufacture and disposal. Stated another way, the inflating gases produced by azide-based gas generants are relatively nontoxic while the metal azides themselves are conversely highly toxic, thereby resulting in extra expense and risk in gas generant manufacture, storage, and disposal. In addition to direct contamination of the environment, metal azides also readily react with acids and heavy metals to form extremely sensitive compounds that may spontaneously ignite or detonate.
In contradistinction, nonazide gas generants, such as those disclosed in U.S. Pat. Nos. 5,035,757 and 5,139,588 to Poole (each herein incorporated by reference), typically comprise a nitrogen-containing nonazide fuel selected from the group of tetrazole compounds and/or metal salts thereof and an oxidizer. “Nonazide” gas generant compositions provide significant advantages over azide-based gas generants by reducing toxicity-related hazards during manufacture and disposal. Moreover, most nonazide gas generants typically supply a higher yield of gas (moles of gas per gram of gas generant) than conventional azide-based gas generants.
However, many nonazide gas generants heretofore known and used produce relatively high levels of toxic substances upon combustion as compared to azide-based gas generants. The most difficult toxic gases to control are the various oxides of nitrogen (NOx) and carbon monoxide (CO). At present, nitrogen monoxide at 75 ppm or less and nitrogen dioxide at 10 ppm or less is generally acceptable. Because the gas generant of the passenger-side airbags is generally four times greater in quantity than that of the driver-side, the need for NOx and CO reduction is most keenly felt when designing passenger-side airbags. Nevertheless, the concern exists for other inflator systems within the vehicle as well.
Reduction of the level of toxic NOx and CO upon combustion of nonazide gas generants has proven to be a difficult problem. For instance, manipulation of the oxidizer/fuel ratio only reduces either the NOx or CO. More specifically, increasing the ratio of oxidizer to fuel minimizes the CO content upon combustion because the extra oxygen oxidizes the CO to carbon dioxide. Unfortunately, however, this approach results in increased amounts of NOx. Alternatively, if the oxidizer/fuel ratio is lowered to eliminate excess oxygen and reduce the amount of NOx produced, increased amounts of CO are produced.
One way to improve the toxicity of the combustion gases is to reduce the combustion temperature that would reduce the initial concentrations of both CO and NOx. Although simple in theory, it is difficult in practice to reduce the combustion temperature and to also retain a sufficiently high gas generant burn rate for practical application in an inflatable vehicle occupant restraint system. The burn rate of the gas generant is important to ensure that the inflator will operate readily and properly. As a general rule, the burn rate of the gas generant decreases as the combustion temperature decreases. By using less energetic fuels, specifically fuels that produce less heat upon combustion, the combustion temperature may be reduced but the gas generant burn rate is also reduced.
Finally, as restrictions on NOx emissions become more stringent, many gas generators, for example those currently used in occupant restraint systems, must be redesigned to reduce NOx emissions. This often requires redesign of contemporary inflators, gas generators, and gas generants. The costs can be substantial. Reducing the NOx without absorbing these costs would certainly be an improvement in the art.
Therefore, a need exists for reducing the toxicity of effluent gases produced by nonazide gas generants without substantially changing the combustion properties or reaction kinetics of the gas generant.